In general, photolithography is a process by which circuit patterns are defined on a semiconductor wafer in high-volume electronics manufacturing. The circuit patterns are transferred from a photomask onto a layer of light-sensitive photoresist on a wafer. For positive photoresist, an application of developer washes away those areas of the photoresist exposed to light. (For negative tone resist the developer washes away the unexposed areas.) The resulting photoresist pattern on the wafer defines the circuit pattern, as metal or other useful films underneath the photoresist will not be cut away in an etching device. On some occasions the patterned photoresist is used as a “mold” into which metal is deposited usually by a plating process, at the completion of the plating process the photoresit is removed leaving the metal posts or plateaus.
Before the pattern is ready to be transferred onto the layer of photoresist, the photoresist is delivered to the wafer surface via spin coat processes. In a conventional spin coat process, a fluid which includes a polymer photoresist and a solvent is deposited in the center of the wafer; subsequently the wafer is rapidly spun. The fluid is pushed to the edge of the wafer by the spinning motion, creating a uniform film as excess solution is pushed beyond the edge. The coated wafer is then placed on a hot plate for a post-apply bake; the remaining solvent is removed by this bake, leaving a firm coat of photoresist. Once the wafer is cooled and the resist sufficiently hardened, the photoresist is ready for exposure.
The thickness of the photoresist layer depends on the particular circuit pattern being transferred. For example, in back-end-of-line (BEOL) processing, which encompasses contact, via, and various metal levels of an electronic device, the photoresist layer typically has a thickness greater than 10 μm. Other levels, such as those developed in front-end-of-line (FEOL) processing, may have substantially thinner photoresist layers.
During the conventional spin coat process, there occurs some evolution of the solvent so that, at the completion of the spin, some drying of the photoresist solution has occurred and the photoresist solution becomes far more viscous and stable than at the beginning of the spin coat process. To reach such stability, conventional spin coat processes for BEOL processes typically take on the order of 60 seconds or more to complete.